This application relates generally to combustors and, more particularly, to fuel delivery systems for gas turbine engine combustors.
Air pollution concerns worldwide have led to stricter emissions standards both domestically and internationally. Aircraft are governed by both Environmental Protection Agency (EPA) and International Civil Aviation Organization (ICAO) standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) from aircraft in the vicinity of airports, where they contribute to urban photochemical smog problems. Most aircraft engines are able to meet current emission standards using combustor technologies and theories proven over the past 50 years of engine development. However, with the advent of greater environmental concern worldwide, there is no guarantee that future emissions standards will be within the capability of current combustor technologies.
In general, one class of engine emissions (NOx) are formed because of high flame temperatures within a combustor. Combustor flame temperature is controlled by increasing airflow during periods of increased fuel flow in an effort to evenly meter combustor flame temperature across the combustor. Known combustors inject fuel through a plurality of premixers that are arranged circumferentially at various radial distances from a center axis of symmetry for the combustor. To achieve a full range of engine operability, such combustors include fuel delivery systems that circumferentially stage fuel flows through the premixers to evenly disperse fuel throughout the combustor.
Such combustors are in flow communication with external boost air systems. As engine power is increased, fuel is injected through premixers at different radial distances. To reduce auto-ignition of fuel, residual fuel is purged from non-flowing premixers with the external boost air system. Because of the various fuel supply and premixer configurations that are used during fuel staging, such external boost air systems are often elaborate and complex. However, despite such complex boost air systems, during fuel stage transitions, pressure decays may occur as a result of the purging. Such pressure decays may cause an overtemperature or overspeed within the turbine.
In an exemplary embodiment, a combustor for a gas turbine engine includes a fuel delivery system that uses circumferential fuel staging and combustor air pressure for purging residual fuel from non-flowing engine components. The fuel delivery system includes a plurality of fuel supply rings and a backpurge sub-system. The plurality of fuel supply rings are arranged concentrically at various radial distances to supply fuel to a turbine engine combustor through a plurality of combustor manifolds and pigtails. The backpurge system uses combustor air to purge fuel from non-flowing fuel supply rings, combustor pigtails, and combustor manifolds. Additionally, the fuel delivery system includes at least two orifices to minimize pressure decays during filling stages.
During engine operation, as power is adjusted, fuel delivery system fuel stages supply fuel to the combustor through various combinations of fuel supply rings. The backpurge system drains and dries residual fuel from the non-flowing fuel supply rings and any associated combustor components. Because the backpurge system uses combustor air at a high pressure and temperature, residual fuel is easily removed and auto-ignition of the residual fuel is reduced. As a result, a combustor is provided that is cost-effective and highly reliable.